We've used GNU Diff as a standard dependency for many of our SCMs for

many years now, as it's been readily available (either built-in or easy

to install) on all target systems for ages. However, that is changing

with the upcoming macOS Ventura release, which will replace GNU Diff

with Apple Diff, a fork of some unknown version of BSD Diff.
To prepare for that, and to provide an opportunity for cleaning up any

code dealing with raw diffs, this change introduces a new

rbtools.diffs.tools module. This provides abstractions around invoking

diff tools and creating a normalized representation of results.
These will eventually be used in place of any execute() or

run_process() call around diff.
To start, this change adds the abstract base class, BaseDiffTool, and

a results class, DiffFileResult.
BaseDiffTool subclasses must set a couple of attributes:

diff_tool_id: A unique string for registration and identification.
name: A human-readable name for the diff tool (used for

  errors/logging).

And implement some methods:

check_available(): Checks for presence/compatibility of the wrapped

  diff tool.
run_diff_file(): Runs the diff tool for two files, converting the

  provided flags to command line arguments, and returning a normalized

  diff result.
get_install_instructions(): Optional, but is used for error messages

  if no diff tool can be found.

Results of a run_diff_file() are represented by a DiffFileResult

object. This wraps a result byte stream that can be used by the caller,

along with some utility methods for getting the original/modified

headers or hunk data in different forms. This also contains state on

whether it diffed a binary file or a text file and whether there were

changes.
Currently, nothing uses this, and GNU Diff support isn't available.

Upcoming changes will flesh this out, providing a diff tool registry,

GNU diff support, SCMClient integration, and finally, Apple Diff

support.

Unit tests pass.
Tested these with some upcoming changes and verified this functionality

works.
Built the docs and checked for obvious errors.